[daveshorts (OP)]

If you are swimming with part of your body above the water, that part does sweat, but what about the part of you thats underwater? Obviously it would not serve the cooling purpose, but if you are hot or swimming hard, can the body tell that it is underwater, and therefore shouldn't waste energy sweating?

[Hadrian]

If you are swimming with part of your body above the water, that part does sweat, but what about the part of you thats underwater? Obviously it would not serve the cooling purpose, but if you are hot or swimming hard, can the body tell that it is underwater, and therefore shouldn't waste energy sweating?

I could be wrong on this but I don't think that skin surface temperature has very little to do with sweating as such. I think that sweating is triggered by our internal temperature which raises as a result of factors such a work rate and the over all external temperature. So you could be running hard in a freezer and still sweat or be doing nothing in a hot tub and still louse lots of body moisture all over including our feet.